The present invention relates to a process for producing an .alpha.-olefin oligomer, and more particularly relates to a process for producing an .alpha.-olefin oligomer comprising mainly of 1-hexene, from ethylene with a high yield and a high selectivity in an industrially useful manner.
It is hitherto known that a oligomerization of .alpha.-olefin such as ethylene is carried out in the presence of a chromium-based catalyst comprising a chromium compound and an organoaluminum compound in combination. For instance, Japanese Patent Publication (KOKOKU) No. 43-18707 discloses a production method of 1-hexene and polyethylene from ethylene by using a catalyst comprising a 6-Group transition metal such as chromium and polyhydrocarbyl-aluminum-oxide.
Further, Japanese Patent Application Laid-Open (KOKAI) No. 3-128904 discloses a trimerization method of an .alpha.-olefin by using a catalyst obtained by previously reacting a chromium-containing compound having a chromium-pyrrolyl bond with metal alkyl or a Lewis acid. Furthermore, Japanese Patent Application Laid-Open (KOKAI) No. 6-239920 discloses a oligomerization method of an .alpha.-olefin by using a catalyst obtained by mixing a chromium compound, a pyrrole-containing compound, a metal alkyl compound and a halide source with each other in a common solvent.
In addition, the present inventors have recently proposed, in Japanese Patent Application Laid-Open (KOKAI) No. 8-3216, an oligomerization method of an .alpha.-olefin by using a chromium-based catalyst comprising a chromium-containing compound, a pyrrole ring-containing compound, a metal alkyl compound and a halide source in combination in such a manner that the chromium-containing compound and the metal alkyl compound are kept in non-contact with each other until these compounds are brought into contact with an .alpha.-olefin. According to the afore-mentioned method, 1-hexene can be produced by subjecting .alpha.-olefin, particularly ethylene, to oligomerization in the presence of the catalyst having a high catalytic activity.
However, in any of these methods, a considerable amount of polymer as by-product have been inevitably produced. Such a production of the polymer as by-product causes problems such as adhesion of the polymer to an apparatus used, necessity of removing the polymer from a solution containing a reaction product, or the like. For these reasons, when it is intended to conduct the oligomerization of .alpha.-olefin in an industrially effective manner, it is important to reduce an amount of the by-product to as low a level as possible.
Further, in the method described in Japanese Patent Publication (KOKOKU) No. 43-18707, a large amount of polyethylene is produced simultaneously with the production of 1-hexene. In the case where the reaction condition is adjusted so as to reduce the amount of polyethylene as a by-product, there is caused a problem that a catalytic activity of the catalyst in the reaction system is deteriorated. In the method described in Japanese Patent Publication (KOKOKU) No. 3-128904, there arises a problem that an amount of the high-molecular weight polymer produced can be limited to a small level but a catalytic activity thereof is insufficient.
In addition, in the method described in the Japanese Patent Application Laid-Open (KOKAI) No. 6-239920, the catalytic activity is also insufficient from a standpoint of industrial-scale oligomerization of .alpha.-olefin, though the method exhibits a high selectivity for 1-hexene. Whereas, in the method described in the Japanese Patent Application Laid-Open (KOKAI) No. 8-3216, the selectivity for 1-hexene is insufficient, though the catalyst used in the method exhibits a sufficient catalytic activity to carry out the industrial-scale oligomerization of .alpha.-olefin.
As a results of the present inventors' earnest studies for solving the above problems, it has been found that by using the chromium-based catalyst formed from at least a chromium compound (a), a nitrogen-containing compound (b) selected from the group consisting of amines, amides and imides, and an alkyl aluminum compound (c) while maintaining a molar ratio of the .alpha.-olefin oligomer to .alpha.-olefin in the range of 0.05 to 1.00, an .alpha.-olefin oligomer such as 1-hexene can be produced with a high yield and high selectivity while extremely reducing an amount of polymer as by-product. The present invention has been based on this finding.